A major cause of birth defects is due to chromosome abnormalities, particularly aneuploidy. An increased incidence of aneuploid babies born to women over 35 years of age was first identified by Penrose in 1933 and now forms the basis for genetic counseling in prenatal diagnosis. While the risk for chromosomal disorder in the liveborn offspring of women ages 35 and older ranges from 1.21% at age 35 to 8.12% at age 46, the contribution of these older mothers to the total incidence of chromosomally abnormal babies is only about 25%, due to the vastly larger numbers of babies born to women under 35 years of age. This means that currently by performing amniocentisis on the approximately 6% of pregnant women who are 35 or older, 25% of chromosomally abnormal fetuses can be detected. Conversly, this also means that about 75% of chromosomally abnormal features are born to the younger age group of women who are not offered genetic amniocentisis.
While using maternal age as a screening parameter is clearly useful, other parameters or tests are needed, particularly biochemical tests which can be offered to pregnant women regardless of age. In this regard Merkatz et al have shown that low levels of maternal serum alphafetoprotein (AFP) are somewhat correlative wit fetal chromosome abnormalities. However, this test appears to have limited wide spread applicability since recent studies have shown that in pregnant women with low AFP levels only about 20% actually carried a chromosomally abnormal fetus.